Re: Proposal: create an Extractor for PartialFunction

I think it's a good idea.  I'd make it more general, actually: call it
`Computes`, and make a (lower priority) version on Function1 also.

You often want to do things like

foo match {
  case bar.Computes(x) if baz(x) => x
  case _ => quux
}

where the only difference with the PartialFunction is that you don't always
need the if clause since it's embedded.

It wouldn't be fast for primitives, for instance, since you always have the
overhead of at least a virtual Some wrapper (and maybe a real one,
depending on the JVM).  But it'd sure be handy.

  --Rex




On Thu, Jul 9, 2015 at 8:06 AM, 杨博 <[email protected]> wrote:

>
> https://github.com/search?q=if+catcher.isDefinedAt&ref=reposearch&type=Code&utf8=%E2%9C%93
>
> There are too many people who have to use case e if
> catcher.isDefinedAt(e) =>
>
> Does that matter?
>
> 在 2015年7月9日星期四 UTC+8下午10:57:40，杨博写道：
>
>> lift and applyOrElse are hard to work with other cases in a match
>> expression, especially when dealing with Exceptions. For example:
>> https://github.com/Atry/memcontinuationed/blob/master/src/main/scala/com/dongxiguo/memcontinuationed/Memcontinuationed.scala#L599
>>
>> 在 2015年7月9日星期四 UTC+8下午10:43:15，Vlad Patryshev写道：
>>>
>>> Here's some discussion of this issue:
>>> http://stackoverflow.com/questions/5902266/inverse-of-partialfunctions-lift-method
>>>
>>> Instead of going back into the world of booleans and conditions, I'd
>>> suggest to think about why there's no such thing already available. Maybe
>>> this is the right direction. applyOrElse is total; if it does not work
>>> for you, lift should. It is pretty scientific, and gets you to a
>>> Kleisli category. All these ifs and match/cases are a little bit like
>>> crutches.
>>>
>>> Thanks,
>>> -Vlad
>>>
>>> On Wed, Jul 8, 2015 at 9:56 PM, 杨博 <[email protected]> wrote:
>>>
>>>> Sometimes, when I create a method, I need an extractor parameter for
>>>> pattern matching, while there is not a generic extractor trait in the Scala
>>>> Standard Library. So we have to use a PartialFunction as the parameter
>>>> instead.
>>>>
>>>>
>>>> When I use the PartialFunction, I had to create pattern matching
>>>> expression like this.
>>>>
>>>>
>>>> input match {
>>>>
>>>>   case x if partialFunction.isDefined(x) =>
>>>>
>>>>     val data = partialFunction.apply(x)
>>>>
>>>>     doSomethingWith(data)
>>>>
>>>> }
>>>>
>>>>
>>>> This approach is not perfect because:
>>>>
>>>>
>>>>    1. partialFunction.isDefined and partialFunction.apply checked x
>>>>    twice, which is slow if isDefined has complex logical, checking
>>>>    whether a file exists for example.
>>>>    2. The if condition around temporary variable x is too trivial.
>>>>
>>>> I think the better approach is converting the PartialFunction to an
>>>> extractor and matching with that extractor
>>>>
>>>>
>>>> implicit final class PartialFunctionAsExtractor[A, B](pf:
>>>> PartialFunction[A, B]) {
>>>>
>>>>   object Extractor {
>>>>
>>>>     def unapply(a: A) = PartialFunction.condOpt(a)(pf)
>>>>
>>>>   }
>>>>
>>>> }
>>>>
>>>>
>>>> input match {
>>>>   case partialFunction.Extractor(data) =>
>>>>     doSomethingWith(data)
>>>> }
>>>>
>>>> Since this is a common approach, I hope adding the
>>>> PartialFunctionAsExtractor to scala.Predef .
>>>>
>>>> I would like to create a pull request if people like this idea.
>>>>
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>>>
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